1. Field of the Invention
The present invention relates generally to lighting control systems and, more particularly, to lighting control systems using zones with multiple occupancy sensors.
2. Discussion of Related Art
Lighting control systems employ sensors and controllers to automatically and selectively power light fixtures on and off, thereby providing light where and when it is needed and reducing lighting in unoccupied areas to achieve energy and cost savings. Many lighting control systems employ occupancy sensors to conserve energy by activating and deactivating light fixtures automatically, depending upon occupancy of areas. Occupancy sensors typically provide a stationary signal (i.e., constant voltage or current level for a predetermined time period) representing occupancy, which is derived from an occurrence of movement. Since an occupant is generally not continuously in motion, a time delay is added to an occurrence of movement to create a period of occupancy. This period of occupancy is assumed to represent an occupied area, such that the light fixtures in that area are activated and held on for as long as the area is occupied. The time delay that is used to create the period of occupancy is a preset time interval that is typically between three and thirty minutes in duration.
Referring to FIG. 1, occupancy sensor devices in a conventional lighting control system are often split into two components, namely, a power pack 110 and an occupancy sensor 120. The sensor 120 receives operating power (on line 140) from the power pack 110 and provides an occupancy signal (on signal line 150) to the power pack 110, the occupancy signal representing occupancy of a monitored area. The occupancy signal on signal line 150 is used by the power pack 110 to control an internal relay 130. The relay 130 closes in response to the occupancy signal to activate a lighting fixture 160 connected through the relay 130.
There are several different types of occupancy sensors used by current lighting control systems, including, for example, passive infrared (“PIR”) sensors and ultrasonic sensors. PIR sensors activate lighting fixtures whenever a moving or additional heat source is detected. Ultrasonic sensors emit ultrasonic vibrations at frequencies of 25 kHz or higher and listen to the return of echoes. If a significant Doppler shift is detected, the ultrasonic sensor indicates a high probability that there is movement in the area. Ultrasonic sensor technology allows continuous detection of moving objects that reflect ultrasonic acoustic energy. The lighting fixtures are then activated in response to the detected movement.